This invention relates generally to methods and apparatus for cardiac CT imaging, and more particularly to methods and apparatus for reducing induced motion artifacts in cardiac CT imaging.
In at least one known computed tomography (CT) imaging system configuration, an x-ray source projects a fan-shaped beam which is collimated to lie within an X-Y plane of a Cartesian coordinate system and generally referred to as the xe2x80x9cimaging planexe2x80x9d. The x-ray beam passes through the object being imaged, such as a patient. The beam, after being attenuated by the object, impinges upon an array of radiation detectors. The intensity of the attenuated beam radiation received at the detector array is dependent upon the attenuation of the x-ray beam by the object. Each detector element of the array produces a separate electrical signal that is a measurement of the beam attenuation at the detector location. The attenuation measurements from all the detectors are acquired separately to produce a transmission profile.
In known third generation CT systems, the x-ray source and the detector array are rotated with a gantry within the imaging plane and around the object to be imaged so that the angle at which the x-ray beam intersects the object constantly changes. A group of x-ray attenuation measurements, i.e., projection data, from the detector array at one gantry angle is referred to as a xe2x80x9cviewxe2x80x9d. A xe2x80x9cscanxe2x80x9d of the object comprises a set of views made at different gantry angles, or view angles, during one revolution of the x-ray source and detector. In an axial scan, the projection data is processed to construct an image that corresponds to a two dimensional slice taken through the object. One method for reconstructing an image from a set of projection data is referred to in the art as the filtered back projection technique. This process converts the attenuation measurements from a scan into integers called xe2x80x9cCT numbersxe2x80x9d or xe2x80x9cHounsfield unitsxe2x80x9d, which are used to control the brightness of a corresponding pixel on a cathode ray tube display.
In one known CT scanner, the rotating gantry rotates at a rate no greater than 2.0 rotations per second, or equivalently, 0.5 seconds per rotation. A typical period for a cardiac cycle is slightly less than one second. Thus, a patient""s heart goes through a substantial portion of its cycle during one gantry revolution. Motion-induced image artifacts result and are a major problem in cardiac CT imaging. In one known system, to reduce artifacts to an acceptable level, an entire scan is performed in a fraction of cardiac cycle with an electron beam CT (EBCT) imaging device. In an EBCT imaging device, x-ray emissions generated by a scanning electron beam are used for imaging. Physical rotation of a gantry is eliminated, and a scan of the electron beam can be completed in as little as 50 milliseconds to essentially completely freeze cardiac motion for coronary imaging. However, EBCT imaging devices remain considerably more expensive than conventional CT imaging devices and are not available in many hospitals. It would therefore be desirable to provide methods and apparatus for reducing motion-induced artifacts in cardiac imaging with conventional CT imaging equipment, or with conventional CT imaging equipment having only inexpensive modifications.
The present invention thus provides methods and apparatus for reducing motion-induced artifacts in CT imaging equipment, including those with a conventional rotating gantry, radiation source, and radiation detector array. For example, in one embodiment, a method for imaging a heart with a scanning computed tomography (CT) imaging system is provided in which a cardiac cycle of a patient is measured; the patient""s heart is scanned with the scanning CT imaging system, at an angular rate asynchronous to the measured cardiac cycle to obtain image data; and an image of the patient""s heart is assembled from chronologically discontinuous segments of the image data. The assembled image is representative of a selected portion of the cardiac cycle, for example, a relatively quiescent portion.
The above-described embodiment provides greatly improved temporal resolution, because a complete data set can be formed from chronologically discontinuous segments of image data that are each obtained during a very short period of time and at equivalent phases of the patient""s cardiac cycle. No special gantry, radiation source, or detector array is required for the CT scanning equipment, yet temporal resolutions comparable to those obtainable with EBCT imaging devices is attainable. The resulting images are useful in medical applications requiring high temporal resolution images, for example, calcification scoring, which requires a high-resolution image of a relatively still heart.